1. Field of the Invention
This invention relates to vortex generators and particularly to an apparatus of vortex generators which generate vorticies across the exhaust ducting of a jet engine, which vorticies cause a mixing of the hot primary fluids with the cooler fan air passing through the engine, thereby resulting in a lowering of the noise emitted by the engine exhaust.
2. Description of the Prior Art
Present low bypass turbofan jet engines produce large amounts of exhaust noise at high power engine conditions. Noise caused by jet engine exhaust is generally considered to be proportional to at least the sixth power of the velocity of the exhaust. In a conventional turbofan engine the velocity of the hot centrally located primary flow exiting from the turbine is much higher than that of the surrounding cooler flow from the fan. In some engines, the flows are initially separated by a wall or splitter, but are allowed to mix in a tailpipe section prior to passing through the nozzle exit plane. In certain turbofan engine installations, such as the Pratt and Whitney Aircraft JT8D Engine installed in a Boeing Company 727 airplane, the tailpipe length is not sufficient to allow satisfactory mixing. The higher velocity of the primary gases exiting the nozzle creates very high levels of jet noise in mixing with the ambient air.
Devices for mixing these two flows within the engine are known. Mixing devices consisting of convoluted ducting, commonly referred to as "lobe type" mixers have been successful in achieving noise reduction, but have been practically unsuccessful because of their relatively high cost (approximately 10 times the cost of noise reduction apparatus according to the present invention) and weight. Further, the lobe type mixer is not adaptable to all engine/airplane combinations such as the above-mentioned JT8D engine/727 airplane combination because the mixer interferes with the thrust reverser of the engine.
Single vortex generators have been proposed as a low cost, light weight solution to provide increased mixing of the two flows without the problems attendant to the lobe type mixer by Birch, et al, in the U.S. Pat. application Ser. No. 754,046, filed Dec. 23, 1976, which is a continuation-in-part of U.S. Pat. application Ser. No. 563,548, filed Mar. 31, 1975, both applications being assigned to the assignee of the present application. However, in the Birch, et al device, most mixing of the fan flow with the primary exhaust flow occurs near the middle region of the flow field by the single vortex generators which are mounted on the splitter. The present invention has been found to provide mixing across the entire engine exhaust area and thus to provide significantly increased noise reduction from the engine exhaust.